The Role of Mitochondria in Dysregulated Insulin Secretion in Obesity

ABSTRACT Hyperinsulinemia precedes the development of Type 2 Diabetes (T2D) and is thought to play a key role in β-cell failure by increasing secretory workload. While hyperinsulinemia is thought to act strictly as a compensatory response to peripheral insulin resistance, recent evidence supports a pathological role by promoting obesity, hyperlipidemia and hepatic steatosis. Numerous studies demonstrate that dysregulated secretion is intrinsic to the beta cell since increased levels of secretion at fasting glucose concentrations persist in islets isolated from obese animals and human. During normoglycemic fasting, an increase in insulin release can occur independently of glucose, to cause an insidious desensitization to insulin actions. Previous studies in obese mice and new studies in pre-diabetic humans reveal that islets can acquire a new form of insulin secretion that is driven by fat, rather than glucose, thereby permitting insulin secretion at fasting glucose concentrations. This form of malfunction is known as Fatty Acid Induced Insulin Secretion (FASIS): the ability of fatty acids to stimulate insulin secretion at non-stimulatory glucose. The molecular determinants of FASIS and its contribution to T2D development are uncharacterized. We hypothesize that mitochondrial fragmentation in beta-cells induced by exposure to high fat is the major event inducing FASIS. Our preliminary data show that fragmentation elevates fat oxidation (FAO) in beta-cell mitochondria, by unleashing fatty-acid import into mitochondria, revealing a new role of mitochondrial architecture in fuel preference. We provide new evidence that high fat exposure induces fragmentation by destabilizing Mitofusin 2 (Mfn2), via a decrease in Ubiquitin C-Terminal Hydrolase L1 (UCHL1) activity. This project will provide the unprecedented molecular mechanism by which FASIS occurs, as well as determining the major effector downstream of decreased UCHL1 activity in T2D. Our aims will: A) Determine the role of mitochondrial fragmentation in the acquisition of FASIS. We will use genetic and pharmacological tools to elongate or fragment mitochondria, and determine the effect on FASIS. B) Determine how mitochondrial shape controls fatty acid utilization in beta cells. We will delineate whether a genetic and metabolic induction of fragmentation is sufficient to allow for uncontrolled CPT1 activity, while forced elongation decreases CPT1-dependent FAO. We will further establish whether FASIS depends on unregulated CPT1 activity and FAO that are induced by mitochondrial fragmentation. And C) Establish the mechanism by which high fat induces mitochondrial fragmentation in beta cells. We will investigate the effect of excess fat and glucose on mitofusin-2 turnover, and follow our preliminary data showing that exposure of beta cells to fatty acids result in decreased levels of UCH-L1, a de-ubiquitinase that controls Mfn2 ubiquitination and turnover, whereby restoring UCH-L1 activity restores Mfn2 levels.